This invention relates to the measurement of a gas mixture for the determination of concentrations of individual predetermined constituent gases and flow rate of the mixture. The invention is especially directed to a method and apparatus for such measurement in which acoustic attributes of the gas mixture and flow thereof are utilized to derive the desired quantities.
One particularly useful application for measurement of gas concentrations and flow rate is in an automobile's evaporative emissions management system. In such a system, fuel vapor is captured in a canister to prevent its release into the atmosphere. To purge the fuel vapor, intake vacuum is typically applied to the canister which draws the fuel vapor out of the canister and into the engine where it is utilized as part of the fuel charge. Canister purge may result in a rich fuel charge if vapor concentrations are heavy resulting in increased exhaust emissions and reduced quality of driveability. A fuel vapor concentration and flow rate sensor may, therefore, be useful for additional control and monitoring of the introduction of recovered fuel vapor into an engine.
It is well known that the velocity (V) of sound propagating through a gas mixture can be expressed as a relationship between the mixture's specific heats at constant pressure (C.sub.pm) and volume (C.sub.vm), average molecular mass (M.sub.m), absolute temperature (T) and the universal gas constant (R) as follows: ##EQU1##
The individual properties of each constituent gas in the mixture are weighted according to the constituent gas volume fraction (x) and summed to arrive at the specific heats and average molecular mass of the gas mixture as follows: ##EQU2##
For a binary mixture of gases, equations (1) through (5) reduce to the following equation in terms of the volume fraction of one of the two gases: ##EQU3## where subscripts 1 and 2 designate the first and second gases, respectively, in the gas mixture.
If a gas mixture is bounded by a vessel, resonant modes exists which are dependent upon the vessel geometry and the sound velocity therein. For a pipe of length (L) closed at both ends bounding a gas mixture, the lowest order resonant mode, or fundamental resonant frequency (F.sub.res) is expressed as follows: ##EQU4## Equations (6) and (7) reduce to: ##EQU5##
A measurement of the fundamental resonant frequency where the two gases are known leaves the first gas volume fraction (x.sub.1) as the only unknown in equation (8). Therefore, a determination of the fundamental resonant frequency indicates the gas volume fraction. Higher order resonant modes also exist which are related to dimensions of the bounding vessel. Lower frequency resonant modes related to the volume of the vessel are also present. In similar fashion, determination of these resonant modes would indicate the gas volume fraction.
U.S. Pat No. 4,380,167 shows an open ended tube device which relies upon velocity of sound through a gas to determine gas concentrations. This device utilizes narrowband ultrasonic signals generated by a transducer which excite the gases at some ultrasonic frequency and requires a tube of specific unit lengths said to be related to the natural resonant frequency wavelength of the particular gas whose fraction is being measured. This device is said to detect resonance by amplitude threshold detection. This device is limited in the gas concentration range it can detect because of its dependence upon the limited narrowband excitation signal developed by an ultrasonic transmitter. It follows then that the device is inadequate to detect wide range variations in gas concentrations such as from 0 to 100 percent.
Prior art devices to measure flow rate of gases based upon sound velocity through the gas have at least a pair of transducers located some distance apart relative to the central axis of a gas carrier tube or pipe, one being downstream from the other. The transducers alternately transmit and receive ultrasonic signals and a flow velocity is derived from the difference in sound propagation times upstream to downstream and downstream to upstream. More complicated devices utilize an upstream and a downstream multiple transducer array wherein each transducer, in turn and to the exclusion of the remaining transducers, acts as a transmitter to all remaining transducers which act as receivers. Such an arrangement is said to improve accuracy of flow measurements since derivations are based upon multiple propagation paths through various flow patterns, thus providing more complete flow profile data. Pertinent references include: U.S. Pat. No. 4,742,717 to Ichino; U.S. Pat. No. 4,663,977 to Vander Heyden; and U.S. Pat. No. 4,462,261 to Keyes et al. These references rely upon transducers to excite the subject gas and appear to be directed toward flow measurement of a single gas or proportionally stabilized mixture of gases.
U.S. Pat. No. 3,580,092 to Scarpa shows a flow monitoring device which secures to an external surface of a pipe to detect ultrasonic noise caused by shear action of a fluid flowing therein. The device relies upon a flow rate sufficient to generate an acoustic signal of adequate intensity to be accurately detected and does not benefit from acoustic reinforcement such as system resonance would provide.